Hi Daniel,
it's been quite a few years since I was on Koh Samui but from what I remember there is quite a built-up area at Chaweng beach that has a big party vibe, with lots of shops, beach bars and so on. If you go there, you should expect a very lively resort with lost of western travellers and drunkenness. The beach is very beautiful and there is good food to be found, although there are some duff restaurants too. A lot of people rent scooters to explore the island; I would advise against this.
I believe there is lots of exploring to do on the rest of the island, but I was young and drunk so I didn't get past Chaweng. Hopefully some other posters can enlighten you on this.